Patent GB-773494-A (GB)
Title: Improvements in or relating to apparatus for measuring dimensions or relative positions
Inventor: FENEMORE RONALD WILLIAM
IPC: G01B11/00
Filed: 1953-05-26
Abstract: 773,494. Electric signalling systems. MULLARD RADIO VALVE CO., Ltd. May 17, 1954 [May 26, 1953], No. 14635/53. Class 40 (1). [Also in Group XL (b)] In apparatus for measuring a dimension or locating the position of an object such as a machine tool accurately the position of a coarse scale carried by the tool is optically compared with a fine scale by counting subdivision of the fine scale from a datum determined by the coarse. As shown (Figs. 7 and 8) the coarse scale is constituted by a screw-thread 18, the section of which is indicated by the line 5 (Fig. 8) the vertical edges constituting the datum marks the precise position of which is to be determined. This scale is traversed by a vertical line of light produced by a source 1, lens 2, mirror 3 and cylindrical lens 4, these last two elements being mounted on a reciprocating carriage. The line of light passes to the coarse thread through the lower half 5<SP>11</SP> of a mask of 5 and to a photo-cell 12 through the upper half. This upper half consists of alternately opaque and transparent divisions, one hundred of the divisions corresponding to the space between two vertical edges of the screw-thread. Thus as the light beam traverses the fine scale photo-cell 12 produces a series of pulses. Between the lower half of mask 5 and the screw thread is a microscope assembly, which images the beam or to the screw-thread with a reduction ratio of about 40 to 1. Photo-cell 22 behind the thread produces an output depending on the position of the screw-thread relatively to the traversing beam. In practice there are more fine subdivisions than correspond precisely to the spacing between two vertical edges of the thread. The output of cell 22 is also shown in Fig. 8, the vertical drop Q1 representing the crossing of one of the vertical edges by the scanning beam. A similar drop Q2 is produced during the return stroke but this is distinguished by differentiating the output of cell 22 when Q1 produces a negative and Q2 a positive pulse. Operation, Fig. 9.-A manual input unit 26 comprising switches 27, 28, representing hundredths and thousandths of an inch respectively is set up to represent table position, further switches 29 being provided if desired to give the coarse setting to a tenth of an inch. This unit determines the initial setting of a counter 34. As soon as scanning commences photo-cell 12 begins generating pulses but until output of cell 22, differentiated at 31, produces negative pulse 62, gate 32 does not open and pulses cannot pass to counter 34. This counter is so arranged as to count to 100 only and successive pulses are subtracted from the setting of the counter. Unit 30 detects the end of the forward scan and shuts gate 32 and triggers a transfer pulse generator 37. This generator closes transfer relays 35 and transfers the scanning count in counter 34 and it is finally displayed as to magnitude and sense on a meter 41. After a delay to allow transfer stages 35 time to open imposed by delay circuit 35, reset pulse generator 39 provides a delayed reset pulse which is fed through the manual input unit to restore counter 34 to the setting of that unit. Modification.-In the arrangement of Figs. 1 to 4 (not shown), an apertured screen is projected past the edge of a main sawtooth scale, the arrangement being that pulses produced by a scanning beam of light passing both the screen and sawtooth edge are counted. However, to take account of pulses produced when the end of the screen is overlapping a tooth other than that to be measured, a continuous train of pulses is generated by a second photo-cell and provided a pulse has been received from both trains simultaneously counting of the first train continues to the end of the train. Specification 777,935, [Group XIX], is referred to.